Let x be the money he invested at 6% interest, and y be the money he invested at 4%.
Now we know that x+y = 29,000, because that is the total amount he invested.
Let's convert the percent annual interest to decimals: 6% = 0.06 and 4% = 0.04. To do this, divide 6% by 100%. The percent...

Multiply both sides by x: 2 = 12x.
Divide by 12 on both sides to isolate our variable x: 2/12 = x
Reducing this fraction by factoring 2 out of top and bottom, we get x = 1/6.

This question uses what's called the Zero Product Property. It means that if (a)(b) = 0, then either a is 0 or b is 0. (They could both be zero too).
So then, y-3 = 0 or y -6 = 0. Can you finish the problem now?

Without any other information, there's nothing you do to simplify this, since a,b,c, and d are all different variables. Is this the entire question?

First, let's distribute the left hand side:
4*3 - 4*2x = -44
12 - 8x = -44
Next, isolate our variable. We're going to do this by subtracting 12 from both sides of the equation.
-8x = -44 - 12 = -56.
Lastly, divide by -8 on both sides to get x completely alone...